In the field of radio communications, Automatic-Repeat-Request (ARQ) is known as one of the coding techniques. ARQ is used together with an error detection technique such as Cyclic Redundancy Check (CRC).
In a case where an error is detected by CRC, the receiver notifies the transmitter of failure in reception of packet data (hereafter, will be also called “data packet” or simply, “packet”) sent by the transmitter through a control channel. When receiving this notification, the transmitter continues retransmission of a packet relating to the above mentioned transmission packet until the receiver succeeds in receiving the packet data.
As leading ARQ schemes, the following three types of schemes are known: stop-and-wait ARQ, go-back-N ARQ; and selective-repeat ARQ. In these scheme, it can occur that the throughput abruptly lowers as a channel error rate incases. Thus, hybrid ARQ (HARQ), in which ARQ is in combination with forward-error control (FEC), is also proposed.
HARQ is one of the indirect link adaptation techniques and is generally used in a physical layer. In HARQ, two types of schemes are known as leading schemes, a scheme called “chase combining (CC)”, and a scheme called “incremental redundancy (IR)”.
In the CC scheme, the transmitter retransmits the same packet as the transmission (coded) packet whose reception by the receiver has been failed. The decoder of the receiver combines the transmission packet failed in its reception with the retransmitted packet by weighting based on, for example, the reception SNR (Signal-to-Noise Ratio). Therefore, the CC scheme makes it possible to obtain a diversity gain in the time domain.
On the other hand, the IR scheme does not retransmit all the redundant information (parity bit) included in a coded packet, as is performed in the CC scheme. Instead, the IR scheme performs retransmission sequentially while increasing a transmission part of the parity bit every when decoding fails.
Further, as to HARQ scheme, HARQ which is called the N-channel stop-and-wait scheme is also proposed to reduce the buffer amount required in the receiver. As this scheme, the unsynchronized HARQ scheme and the synchronized HARQ scheme are known.
The N channel is a logical channel (hereinafter, will be also called the “HARQ channel”). In the unsynchronized HARQ scheme, the N channel can be identified by the number of the sub-channel which is definitely assigned. On the other hand, in the synchronized HARQ scheme, the N channel can be identified with, for example, predetermined sub-frame timing (time slot).
[Non-patent Document 1] S. Lin and D. J. Costello, JR, Error Control Coding, Fundamentals and Application, Prentice-Hall, Inc, Englewood Cliffs, N.J., 1983.
[Non-patent Document 2] 3GPP TR 25.848 V4. 0. 0, March 2001.
[Non-patent Document 3] J. Wu, W. Tong, and J. Li, “Non-complete puncture based re-transmission for HARQ”, C50-20011105-025, 3GPP2 TSG-C WG5, Nov. 5, 2001.
[Non-patent Document 4] J. Wu, W. Tong, and J. Li, “Simulation Study on NCP-HARQ”, C50-20011203-022, 3GPP2 TSG-C WG5, Dec. 3, 2001.
[Non-patent Document 5] Xiao, L.; Fuja, T. E.; Kliewer, J.; Costello, D. J. Jr. “Nested Coded with Multiple Interpretations”, ISS 2006 40th, 22-24 Mar. 2006.
FIG. 11 illustrates an example of HARQ processing based on the synchronized N-channel stop-and-wait scheme in a case where N=4. FIG. 11 exemplifies a manner in which four packets #1 through #4 are sent in the time of 4TTI by use of four parallel HARQ channels.
Each packet is sent without waiting for transmission of other packets as far as a packet to be sent is present so that the HARQ channel is occupied by transmission packets throughout the time. In the present example, the positions at which retransmission occurs are limited to the positions expressed by, for example, m+k·N (m is the number of the HARQ channel of the retransmission packet to which an attention is paid; k is a positive integer). Accordingly, if this HARQ channel is assigned to another user (UE), there is a possibility that retransmission is delayed.
The N-channel stop-and-wait HARQ scheme is suitable for a lot of standards such as 3GPP2, 3GPP, and IEEE802.16, and it leaves much to be improved in circumstances such as that in which UE moves in low velocity.
That is, in a case where UE moves in low velocity, it is not always necessary to send a normal packet that is requested to be retransmitted in HARQ retransmission. To send a normal packet, in spite of the above fact, leads to unnecessary redundant transmission, so that it can cause a waste of a lot of channel resources.
FIG. 12 illustrates an example of a relationship between mutual (cooperative) Information (MI) and SINR (Signal-to-Interference and Noise Ratio) per symbol.
As exemplified in FIG. 12, SINR necessary in HARQ transmission can be significantly smaller than SINR in a case where a normal packet is sent. The operation point of HARQ is generally set to a Block Error Rate (BLER) of 10%.
Therefore, in a case where UE is under circumstances in which fading variation is not much, the energy necessary for restoring a packet can be small. When this energy is converted in terms of the SNR standard, it can be smaller than 1 dB in its average value.
From such a point of view, a proposition called “NCP” (Non-complete Puncture) based HARQ made for the 1×EV-DV (1× Evolution-Data and Voice) standard in the year of 2001 is publicized. This proposition mainly relates to reduction of the waste of channel resources in HARQ process.
In this NCP retransmission scheme, retransmission packets are dispersed to multiple new packets and punctured so that the retransmission packet occupies a small part of the payload of the new packets. As a result, it becomes possible to significantly increase the throughput of the whole. In this instance, on the receiver, a retransmission packet and a new packet are decoded separately.
Further, recently, the nested coding scheme employing multiple interpretations is proposed. According to this scheme, the multiple packets are coded separately, and each of the thus coded packets is sent out after being subjected to an operation of exclusive OR (XOR) in a physical layer.
However, the above described previous techniques remain at the capability that a retransmission packet is included in the payload of multiple new packets or that an XOR operation of individual packets is performed.